Sintered photoconducting layers



Oct. 2, 1956 s. M. THoMsEN SINTEIRED PHOTOCONDUCTING LAYERS Filed Dec.3, 1954 5/L Vie P4575 ELETFODES caff.' 00 (0, 000

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INVENTOR.

United States Patent SINIERED PHo'roCoNDUCTING LAYER@ Soren M. rIhomsen,Pennington, N. J., assigner to Radio Corporation of America, acorporation of Deiaware Application December 3, 1954, Serial No. 473,0019 Claims. (Cl. 201-63) This invention relates to sinteredphotoconducting layers which are particularly useful in gap type andarea type photocells. The invention includes methods for preparingsintered photoconducting layers and devices utilizing the sinteredphotoconducting layers of the invention.

A photoconductive device is one which displays a reduced resistance toelectric current ow when irradiated with light. In its simplest form, aphotoconductive device comprises a body of photoconductive material anda pair of electrodes attached thereto. When a voltage is applied to theelectrodes, the device displays a decrease in electrical resistance whenthere is an increase in the intensity of light irradiating the device.An amount of electric current ows through the device which is a functionof this electrical resistance.

Ideally, a photoconductive device is a perfect insulator when light towhich it is sensitive is absent, and is a perfect conductor when lamaximum intensity of light to which it is sensitive is present.Actually, a photoconductive device behaves as a high resistanceconductor when light to which it is sensitive is absent and behaves as alower resistance conductor when light to which the device is sensitiveis present.

The difference in conduction produced by the presence of a unitvariation of light intensity is referred to as the photosensitivity ofthe device. The measure of photosensitivity is in terms of photocurrentunder standard conditions. The current passed by the device in darknessis referred to as the dark current, the current passed when the deviceis irradiated is referred to as the light current and the dierencebetween light current and dark current is referred to as thephotocurrent.

One type of photoconductive device comprises a single crystal of aphotoconductive material and electrodes attached to the crystal. Suchsingle crystal photocells exhibit large photocurrents and high ratios oflight current to `dark current. However, the crystals are usually smallin size and, consequently, the total current passed by a single crystalis small. When greater currents are passed through the crystal, thecrystal heats up and the photosensitivity of the crystal is reducedeither temporarily or permanently. Furthermore, photoconductive crystalsare dii'licult to grow and are fragile. Thus, the expense of manufactureand maintenance often prohibits the use of single crystal photocells.

Another type of photoconductive device comprises a body includingfinely-divided photoconducting powder particles and electrodes attachedto said body. The body may include, for example, an unbondedphotoconducting powder or a photoconducting powder mixed with a bindersuch as a synthetic resin. Such powder photocells exhibit a broader bandof spectral response than single crystal photocells. In addition powderphotocells may be prepared in any desired size, shape or currentcarrying capacity. However, these powder-type devices have had thedisadvantage of low photosensitivity, and relatively high resistancewhen the device is irradiated with light to which it is sensitive.

The low-photosensitivity and high resistance of powder photocels isgenerally attributed to the large number of electrical barriers existingbetween the electrodes. The electric current passing between theelectrodes must travel through chains of powder particles. Theresistance due to poor electric contact between adjacent particles ismultiplied by the number of particles in the chain, partly or completelymasking the photosensitivity of the volume of each particle by limitingthe maximum amount of current that can be passed by each chain ofparticles and by heating the particles during the flow of electriccurrent.

An object of the invention is to provide improved photoconductivebodies.

Another object is to provide photoconductive layers having relativelyhigh photosensitivities.

Another object is to provide improved photoconductive devices comprisingthe improved photoconductive bodies of the invention.

A further object is to provide methods for preparing the improvedphotoconducting bodies of the invention.

The photoconducting bodies according to the invention comprise asubstantially continuous polycrystalline layer of interlockedphotoconducting crystals. The crystals may comprise, for example, apredominant proportion of a substance selected from the group consistingof selenides, sulphides, and sulphoselenides of cadmium havingincorporated therein activator proportions of a halide and activatorproportions of a metal selected from the group consisting of copper andsilver.

The devices according to the invention comprise a substantiallycontinuous polycrystalline layer of photoconducting crystals accordingto the invention and at least one electrode attached thereto.

A method for producing a photoconducting layer according to theinvention comprises forming a stratum including particles of a materialselected from the group consisting of sulphides, selenides andsulphoselenides of cadmium, recrystallizing said material in a moltensolvent to a desired range of particle sizes, incorporating into saidrecrystallized material activator proportions of a halide and activatorproportions of a metal selected from the group consisting of copper andsilver and evaporating said molten solvent, thereby producing asubstantially continuous layer of interlocked crystals ofphotoconductive material.

By producing a substantially continuous layer of interlocked crystals ofphotoconductive material, the photosensitivity existing in the volume ofeach particle is unmasked and may be observed and utilized for purposesheretofore believed impractical. Such a layer is simple to prepare,reproducible and the devices prepared therewith are rugged and weatherresistant.

The invention will be more fully described in the following detaileddescription when read in conjunction with the drawing in which:

Figure 1 is one embodiment of a photocell according to the invention.

Figure 2 is a series of spectral response curves for typical photocellsprepared according to the invention and Figure 3 is a second embodimentof a photocell according to the invention.

Similar reference characters are used for similar elements throughoutthe drawing.

Example ].-An intimate mixture of grams of cadmium sulphide, 10 grams ofcadmium chloride, 1.7 milliliters of 0.1 M copper chloride and 500milliliters of water is prepared. This mixture may be prepared in ablender such as is used for mixing powder with water.

The yellow, viscous liquid mixture is applied, as by Table Compositionin ii isn C1 OD Curve CdS: Cu (0.001) 0. 0001 3 300 10-4 10-7 3l CdS: Cu(0.0001)...... 0. l 30 1,000 10'3 l0" 33 CdS@ :Cu (00001)... 0. 0001 3003,000 2 10"l 35 Another type of photocell comprises a transparentconducting layer on a glass substrate over which the photoconductinglayer of the invention is formed. Such a transparent conductive coatingmay abe prepared by exposing heated glass to the vapors of silicon, tinor titanium chloride and afterwards treating the coating thus formed inla yslightly reducing atmosphere. In some cases the glass plate may betreated with 1a mixture of vstannic chloride in absolute alcohol andglacial acetic acid. The electrodes may be in any desired configuration,for example, the electrodes may comprise a simple gap structurecomprising two spaced electrodes.

Referring to Figure 3, 'a photocell of the invention may compriseconducting areas 25 on a glass plate A23 in the configuration of twoelectrodes having la series of interdigitated [fingers extending so thatthe electrodes are equidistant from one :another at every point. lSuch astructure provides uniform gap width and :a relatively long gap lengthfor a .given area. A photoconductive layer 21 of the invention is nowformed on top of the electrodes 25.

The photoconducting layers of the invention may be used in simplephotoconductive devices or in more complicated devices including otherstructures such -as electroluminescent materials and in conjunction withtelevision pickup tubes including cathode ray scanning means.

The sintered photocells of the invention have the advantage over singlecrystal photocells in that they are cheaper and easier to prepare, aremore rugged, exhibit a panchromatic response to light, may be made inany desired size or shape, and may be designed to handle large currents.The sintered photocells of the invention have the advantage over powderphotocells in that they are cheaper and easier to prepare, exhibit agreater response to light at the blue end of the spectrum, exhibit ahigher speed of response to light and exhibit greater photosensitivityat lower voltages. The sintered photoconductive layers of the inventionhave the advantage over presently used vid-icon targets of greaterphotosensitivity rand are easier to prepare.

What is claimed is:

l. A method for producing a sintered photoconductive layer comprisingforming a stratum including particles of a substance selected from thegroup consisting of sulphides, selenides and sulphoselenides of cadmium,recrystallizing said substance in said layer in a molten solvent,incorporating into said recrystallized substance, activator proportionsof a member of the group consisting of copper and silver, removingsubstantially all of said molten solvent and sintering saidrecrystallized substance, thereby producing a substantially continuouslayer of interlocked crystals of photoconductive material.

2. A method for preparing a photoconductive layer comprising forming astratum including particles of cadmium sulphide, recrystallizing saidcadmium sulphide in a molten solvent, incorporating into saidrecrystallized substance activator proportions of chloride and copper,evaporating substantially all of said molten solvent and sintering saidrecrystallized cadmium sulphide thereby producing a substantiallycontinuous layer of interlocked crystals of photoconductive material.

3. A method for preparing a photoconductive layer comprising forming astratum including particles of cadmium selenide, recrystallizing saidcadmium selenide in a molten solvent, incorporating into saidrecrystallized substance activator proportions of chloride and copper,evaporating substantially all of said molten solvent and sintering saidrecrystallized cadmium selenide thereby producing a substantiallycontinuous layer of interlocked crystals of photoconductive material.

4. A process for producing a photoconductive layer comprising coating asubstrate with an intimate mixture of about parts by weight cadmiumsulphide, 10 parts by weight cadmium chloride, and 0.01 parts by Weightof copper and then firing said coating at about 600 C. in an atmospherethat is inert to said coating until a substantially continuous layer ofinterlocked crystals of photoconductive material is produced.

5. A substantially continuous polycrystalline layer of interlockedphotoconducting crystals of cadmium sulphide containing activatorproportions of chloride and copper.

6. A substantially continuous polycrystalline layer of interlockedphotoconducting crystals of cadmium selenide containing activatorproportions of chloride and copper.

7. A substantially continuous polycrystalline layer of interlockedphotoconducting crystals, said crystals comprising a substance selectedfrom the group consisting of sulphides, selenides and sulphoselenides ofcadmium, said crystals having incorporated therein activator proportionsof a halide and having activator proportions of an element selected fromthe group consisting of copper and silver.

8. A photoconductive device comprising a substantially continuouspolycrystalline layer of interlocked photoconducting crystals of cadmiumsulphide containing activator proportions of chloride and copper and atleast one electrode attached to said layer.

9. A photoconductive device comprising a substantially continuouspolycrystalline layer of interlocked photoconducting crystals, saidcrystals comprising a substance selected from the group consisting ofsulphides, selenides and sulphoselenides of cadmium, said crystalshaving incorporated therein activator proportions of a halide and havingactivator proportions of an element selected from the group consistingof copper and silver and at least one electrode attached to said layer.

References Cited in the file of this patent UNITED STATES PATENTS2,582,850 Rose Ian. 15, 1952 2,629,039 Shoemaker Feb. 17, 1953 2,651,700Gans Sept. 8, 1953 2,668,867 Ekstein Feb. 9, 1954 2,706,792 Jacobs Apr.19, 1955

